1. Field of the Invention
The present invention pertains to methods of and means for raising the concentration of a dissolved gas in a body of liquid, and more particularly, it pertains to a means for increasing the dissolved oxygen level of the sewage within the mixed liquor tank in an activated sludge sewage treatment process.
2. Description of the Prior Art
In the treatment of sewage by the conventional activated sludge process, the bacterial colony, known as the bio-mass, converts the carbon-hydrogen compounds within the sewage to more bio-mass, carbon dioxide and water. As is well known, oxygen is an essential ingredient in this fermentation process. During the course of treatment of the sewage, enough oxygen must be dissolved therein to oxidize essentially all of the biodegradable matter.
The rate at which oxygen is dissolved in water at any given instant is a function of (1) interfacial contact area and (2) the driving force, which can be defined as the difference between the concentration of oxygen in water at saturation and the concentration at that instant (C.sub.s -C). Interfacial contact area between the oxygen and the water is usually obtained by agitation of the water, a process which requires the input of energy and is therefore an important cost in the activated sludge treatment of sewage. In conventional treatment of the sewage with air, i.e., where oxygen in the form of air is to be dissolved in the sewage, (C.sub.s -C) is seven or eight milligrams per liter and interfacial area must be provided sufficient to dissolve the required oxygen fast enough to meet the demand of the bio-mass. When (C.sub.s -C) is maintained higher, the agitation requirement is lower, but, of course, the lower concentration of oxygen in the mixed liquor requires a greater amount of time to be expended in the treatment process. The oxygen concentration at saturation, C.sub.s, is proportional to the partial pressure of oxygen in the contacting gas. Oxygen pressure in air is 0.21 atmospheres, and C.sub.s is therefore about 9 milligrams per liter at 70.degree. F. When pure oxygen is used (partial pressure one atmosphere), C.sub.s is about 43 milligrams per liter, and rate of dissolution of the oxygen into the sewage is about five times as high as it is when air is being used--other factors being equal. Thus, when using pure oxygen, the costs for agitating the mixed liquor are reduced; however, the cost of producing oxygen from air for use in the sewage treatment process usually offsets this savings.
One method of hastening the sewage treatment process and reducing the agitation costs would be to use pure oxygen at elevated pressures wherein C.sub.s will be proportionately higher. For example, at a pressure of 200 psig (14+ atmospheres), C.sub.s is about 600 milligrams per liter. Use of oxygen under such increased pressures would greatly reduce the agitation and time requirements for the sewage treatment process, but the cost of constructing mixed liquor tanks to handle these higher pressures has proven, in general, to be prohibitive. Prior art patents which show sewage treatment processes wherein oxygenation is carried out under increased pressures include the prior patents to Othmer U.S. Pat. Nos. 3,772,187; Martin 3,779,913; Neel 3,808,123; Cervantes 3,804,055; and Zimmerman 2,665,249.
Another form of the use of pressurized oxygenation in a sewage treatment process is disclosed in U.S. Pat. No. 2,809,933 to Halvorson. In the process disclosed in this patent oxygen under superatmospheric pressure is dissolved within sewage influent in a closed pressure treatment tank. After an initial treatment time, comprising a time period of from 2-30 seconds, the immediate oxygen demand of the sewage is satisfied and a valve is opened from the treatment tank to discharge the oxygenated influent to a trickling filter or other sewage treatment system employing aerobic bacteria.
Finally, systems have been proposed for recycling a portion of the sewage which is undergoing treatment in the mixed liquor tank and for inserting oxygen into this recycled stream. Such a system is shown, for example, in U.S. Pat. No. 3,477,947 to Kappe. Other systems of the same general type, and generally known as bio-precipitation processes, are described on pages 127-134 and 176-188 of "Applications of Commercial Oxygen to Water and Waste Water Systems", Water Resources Symposium No. 6, Edited by Richard E. Speece and Joseph F. Malina, Jr., published by the Center for Research in Water Resources, University of Texas at Austin, Tex. (1973).